Automatic gain adjusting device for reflection seismographs



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' AUTOMATIC GAIN ADJUSTING DEVICE FOR REFLECTION SEISMOGRAPHS Filed June 26, 1936 5 Sheets-Sheet l 5. TSEIBMOQRAPH recur-v AMPL/Tu i I I l l I I I I I I I I I I IIlllIIIliIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII WZMW Dec. 26, 1944. 2,366,043

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w. D. MouNcE AUTOMATIC GAIN ADJUSTING DEVICE FOR REFLECTION SEISMOGRAPHS 3 Sheets-Sheet 5 Filed June 26. 1936 HMQROPQ 20 Mk twxyr k.

Patented Dec. 26, 1944 AUTOMATIC GAIN ADJUSTING DEVICE FOR REFLECTION SEISMOGRAPHS Whitman D. Mounce, Houston, Tex., assignor to Standard Oil Development Company, a corporation of Delaware Application June 26, 1936, Serial No. 87,418

g to pick the time of arrival of any of the series Claims.

This invention relates to improvements in the recording of seismic vibrations. More particularly it relates to an improved method and apparatus for adjusting the recorded energy from an explosion of dynamite in the ground.

The invention will be fully understood from the following description taken in connection with the accompanying drawings, in which lattar- Fig. l is a representation of the seismograph record ordinarily obtained.

Fig. 2 is a composite representation of the average amplitudes of the energy received on a large number of typical records similar to that illustrated in Fig. 1 and extending from the point A to the right-hand end of the record of Fig. 1.

Fig. 3 is a graphical representation of the adjustment of the gain of the receiving system necessary to receive the energy at the recorder with constant average amplitude from the point A to the right-hand end of the record of Fig. 1.

Fig. 4 is a diagrammatic representation of a preferred form of apparatus for carrying out the invention, and

Fig. 5 is a representation of the seismograph record obtained with this invention.

In seismic prospecting it is customary to create seismic energy at a source and to receive the seismic energy at a plurality of detectors spaced from the source. The seismic energy received at the detectors is transformed into pulsations of electric energy, which are then amplified and recorded. The average amplitude of the energy as received by the detectors or seismographs falls oif rapidly with time after the arrival of the first impulses from the shot.

Referring to Fig. 1, a record obtained when using six detectors is illustrated in which the lines 3 designate time intervals, and the traces 5 to Hi inclusive represent the amplitudes of motion of the recording elements of the detectors as determined by the motion of the ground at the respective positions of the detectors. As shown on the record, the amplitude of the energy received at the first part of the record designated 11-13 is so great that the traces representing the amplitudes of each detector cover the traces of the adjacent detectors. As a result, it is extremely difiicult to pick the time of arrival of any one of the series of impulses from the shot within this interval of time A--B. From the points designated B-C on the record, the received energy from the charge is of the desired amplitude, thereby enabling the observer of impulses with a high degree of accuracy. From the point designated C to the end of the record, the amplitude of the received energy has fallen oil to such a degree that the information contained on the record is of little practical value. Consequently, it has been the practice to shoot at least two more charges of dynamite, one of a smaller quantityof explosive than the first to reduce the recorded amplitude, which will enable the observer to decipher the information contained in the interval of time A-B, and another larger charge, which will increase the recorded amplitude from C to the end of the record, whereby the arrival time of any of the series of impulses in the interval of time may be observed with the necessary degree of accuracy.

Observation of a large number of reflection seismograph records similar to the one illustrated in Fig. 1 shows that the average amplitude of the received energy falls off with time after the first impulse in a manner graphically illustrated in Fig. 2. Here the. curve l4 represents the manner in which the average amplitude of the recorded energy decreases with time after the arrival of the first impulse from the explosion at point A to the end of the record.

It is an object of this invention to introduce into the receiver system an automatic amplitude adjusting device which operates simultaneously on each of the several detector-amplifiergalvanometer combinations of a seismic receiving station in such a way that the amplificatlon of each combination is changed with time as shown in Fig. 3, in which the curve I6 is the reciprocal of the curve It shown in Fig. 2.

A preferred form of apparatus for accomplishing this adjustment is illustrated in Fig. 4, in which the numeral 20 represents a multistring galvanometer with a light source 2| and recorder 23. The light from light source 2| passes through slots 24 and 25 and falls upon the film of recorder 23. The galvanometer is provided with strings 21 to 32 inclusive, which are disposed in the path of the light from the source 2|. The strings 21 to 32 are connected respectively to amplifiers 35 to 40 inclusive, as illustrated in Fig. 4. The amplifiers 35 to 40 inclusive are connected to detectors 4| to 46 inclusive. The detectors are ordinarily disposed at or near the surface of the earth inposition to receive seismic vibrations from a suitable shotpoint, not shown, at which a charge of dynamite may be exploded. The detectors M to 48 are operative to transform the seismic vibrations a trigger 55 pivotally mounted at 55a. I ment battery for the thyratron 53 is designated into corresponding pulsations of electric energy which after amplification are caused to deflect the strings 21 to 32. As shown in the diagram, detector 4| is operatively connected to amplifier 35 and amplifier 35 is connected directly to recorder string 21. Similarly, detectors 42 through 45 are connected respectively to amplifiers 35 spection of Fig. 5 that the amplitudes of each trace during the time interval from A to B have been reduced so that the traces of adjacent detectors do not overlap each other. The traces from B to C have been substantially unaffected The amplitude of the traces from C to th end cated in Fig. 3.

is received by the detector 4| and amplifier as, a

positive voltage is applied to the grid of a thyratron 53 which latter sends current from a battery 54 through relay coils 55, thereby releasing The fila- 55b. The electromagnetic field of the galvanometer is varied as a predetermined function of time by varying the resistance 50 as follows. A cylinder 51 contains a suitable liquid 53, and a piston 59 is mounted for reciprocation within this cylinder. The piston 59 carries a piston rod 50 which is mounted for reciprocating motion in a guide iii. The piston. rod 50 is provided with a The various detectors shown in Figure 4 are numbered in order to facilitate the tracing of the several circuits involved. The outputs of the amplifiers are connected by conductors 54 to strings 21-32 inclusive, of the galvanometer. The

' several amplifiers are grounded by conductors 55.

contact arm 62 one end of which bears slidably against the variable resistance 50 and is connected in the circuit through the coil 48. The latch 55 normally holds the contact arm 52 at the position of greatest resistance of the resistor 50, designated D. A tension spring 53 is connected to the contact arm 52 and to the guide 5| to normally exert pull upon the contact arm to .draw it to position E in which the resistor 53 The spring 53 accomhas a decreased value. plishes this function when the latch 55 releases the contact arm 52.

The piston 59 is caused to move through the cylinder 51 with a uniform predetermined velocity as follows: The piston 59 is provided with an orifice 65 to permit flow of the liquid 58 therethrough to opposite sides of the piston. The pressure upon the piston exerted by the spring 53 forces the liquid 58 through the orifice 56, and by proper choice of the spring 53, liquid 58 and diameter of the orifice 55, th piston 59 moves through the cylinderwith a uniform predetermined velocity.

For proper adjustment, the time required. for the piston 59 to move the length of resistance 50 is chosen to be the interval of time A to the end of the record shown on Fig. l. The resistance 50 is made by winding wire of suitable resistance over a form, the shape of which is determined by the slope of the curve l5 shown in Fig. 3, so as to decreasefthe amount of resistanc in proportion to the decrease of average amplitude of the seismic energy. Hence, when the time required for the contact arm 52 to move the length of the resistance, 50 is chosen to be the same as the total time shown in Fig. 2, the sensitivity of the galvanometer 20 varies with time 'as shown in Fig. 3 and the amplitude of each trace is approximately constant over the entire record.

The time scale of Fig. 5 is the same as that of Fig. l, and in the record illustrated the points A, B and C in Fig. 5 correspond with time to the points similarly designated on the record illustrated in Fig. 1. It will be seen from in.

Amplifier 35 has connected to its output an additional conductor I3 which is connected to the grid of thyratron 53. The plate of the circuit is connected by conductors 51 to relay coils 55 which, in turn, are connected to trigger 55. When contact arm 52 is in contact with trigger 55 the current passes from trigger 55 through contact arm 52 and conductor 58 to ground wire 650i amplifier 35. When trigger 55 is withdrawn from contact arm 52 the plate circuit is broken. The cathode of thyratron 53 is connected to ground wire 58 by conductor 59.

The negative side of battery 49 is connected by conductor 10 to ground wire 58. The positive side of the battery is connected to conductor 'i l which is wrapped around a core of the galvanometer to form field winding 48 and is connected to contact arm 52 through resistance 50 and thence to the ground through conductor 58. p

The various strings of the galvanometer are connected to conductor 12 which is connected to conductor 10 and thus to conductor 58 and the ground.

While automatic means have been shown and described for simultaneouslyvarying the ampliv tude of movement of the responsive elements as traced upon the recording sheet, it will be understood that the operatim may be done manually by the operator and some of the advantages of the invention will be retained.

Various changes and alternate arrangements may be made within the scope of the appended claims, in which it is my intention to claim all novelty inherent in the art as broadly as the prior art permits.

Iclaim:

1. In seismic prospecting apparatus for receiving seismic waves the amplitudes of which decrease as a function of time, and converting said waves into electrical pulsations of corresponding amplitudes, means for conducting said pulsations to a recorder including an electromagnet magnetized by current flowing in a coil of wire, elements adapted to respond to said impulses so disposed in the field of the electroma net that their response is controllable by the magnetic field resulting from the current in said coil of wire, a resistance for controlling the current flowing in said coil of wire and means for decreasing this resistance at a rate which is in substantially inverse proportion to the rate of decrease of the amplitudes of the received waves,

whereby the average amplitudes of the recorded waves are maintained substantially constant.

2. Seismic prospecting apparatus comprising means for converting seismic waves, the amplitudes of which decrease as a function of time, into electrical pulsations of corresponding amplitudes, a recorder connected to said means having in an electromagntic field a plurality of re-' sponsive elements adapted to be activated proportionately to the amplitudes of the pulsations and means for increasing the electromagnetic field and, thereby, the responsiveness of said elements at a rate corresponding to the rate of decrease of the amplitudes of said pulsations.

3. Seismic prospecting apparatus comprising means for receiving seismic waves, the amplitudes of which decrease as a function of time, and converting them into electrical pulsations of corresponding magnitudes, a recorder connected to said means having in an electromagnetic field a plurality of responsive elements adapted to be activated proportionately to the amplitudes of the pulsations, controlling means associated with said recorder for normally maintaining said electromagnetic field at a. reduced strength and comprising means for increasin the strength of said field, and means actuated by the first electrical impulse transmitted by said receiving means for setting said field-strength-increasing means into operation, whereby the strength of said magnetic field and, consequently, the responsiveness of said responsive elements increases as the amplitudes of the electrical impulses decrease.

4. Seismic apparatus, according to the preceding calim, in which means are included for controlling the rate of increase of strength of the magnetic field.

5. Seismic prospecting apparatus comprising means for receiving seismic waves, the amplitudes of which decrease as a function of time, and converting said waves into electrical impulses of corresponding amplitudes, a string galvanometer connected to said means having strings adapted to be activated proportionately to the amplitudes of the pulsations and a coil adapted to set up an electromagnetic field around said strings to control their sensitivity, aresistance connected'to said coil, a sliding arm on said resistance, a latch for normally holding said sliding arm in a position in which the value of said resistance is high, means actuated by the first electrical impulse transmitted'by said receiving means for tripping said latch and means for sliding said arm over said resistance toward its low point at a rate such that the value of said resistance decreases as the amplitudes of the electrical pulsations decrease, whereby the strength of said electromagnetic field and, thus, the sensitivity of the strings of the galvanometer, increase as the amplitudes of said pulsations decrease.

WHITMAN D. MOUNCE. 

